2. Until version 59 (exclusive): this feature is behind the dom.webcomponents.enabled preference (needs to be set to true) and the dom.webcomponents.customelements.enabled preference (needs to be set to true). To change preferences in Firefox, visit about:config.

3. From version 59: this feature is behind the dom.webcomponents.shadowdom.enabled preference (needs to be set to true). To change preferences in Firefox, visit about:config.